I can certainly understand the reluctance to consider either a small Lipo pack, or a BEC, and I don't use either myself so it's not something I actually advocate, but what if the two were combined to create a redundant system?

What I'm thinking about here is a system that uses a BEC to power the receiver and the servos, and a small backup battery that would provide enough power to save the plane if anything happened to the main batteries or the BEC. The idea is that during regular operation the BEC would keep the backup battery charged so you wouldn't have to charge it between flights. I'm not an electrical engineer, but I'm sure that the BEC would have to be combined with a regulator, and also some circuitry to protect the backup battery from being overcharged. There's also the additional question as to how much weight such a system could save, but even if it's a wash, you'd have a system that wouldn't require you to watch your receiver battery all the time.